1. Field of the Invention
This invention relates to a lens actuator for use in the recording/reproducing pickups of various apparatuses such as an optical video disk player, a compact digital audio disk player and an optical data memory.
2. Related Background Art
Known as the above-described lens actuator is one as described, for example, in U.S. Pat. No. 4,571,026. FIG. 1 of the accompanying drawings is a plan view showing an example of such prior-art lens actuator, and FIG. 2 of the accompanying drawings is an exploded perspective view of the same.
Referring to FIGS. 1 and 2, an objective lens 101 is held by a lens holder 102. The lens holder 102 is provided with bearings not shown and is supported by a support shaft 103 for rotation about this support shaft and for sliding in the direction of the center axis of this shaft. A balancer 109 for keeping weight balance with the objective lens 101 is mounted at a position on the lens holder 102 which is symmetrical with the objective lens 101 with respect to the shaft.
A focusing coil 105 and a tracking coil 104 are mounted on the cylindrical side of the lens holder 102. A magnetic field is applied to these coils by a magnetic circuit comprised of a magnet 106, an external yoke 107, an internal yoke 108 and a bottom yoke 111. The lens holder 102 has two openings 102a in which the internal yoke 108 is inserted.
In the lens actuator constructed as described above, when an electric current is supplied to the focusing coil 105, a vertical drive force is generated and the lens holder 102 slides relative to the shaft 103. Along therewith, the objective lens 101 moves in the direction of the optic axis thereof, and the focus position of a light beam condensed on the optical disk through this lens varies relative to the surface of the disk. If the electric current supplied to the focusing coil 105 is made to correspond to a focusing error signal, focusing servo will be effected by the movement of said lens.
On the other hand, when an electric current is supplied to the tracking coil 104, a force in the direction of rotation about the shaft 103 is generated and rotatively drives the lens holder 102. Along therewith, the objective lens 101 moves in a direction orthogonal to the optic axis, and the light beam travels across tracks formed on the disk. Accordingly, by the electric current supplied to the tracking coil 104 being made to correspond to a tracking error signal, tracking servo is effected.
However, a problem as will hereinafter be described has arisen in the lens actuator as shown in FIG. 1. For example, when an optical disk whose surface is warped and which has caused eccentricity is rotated at a high speed, the actuator is driven at a high frequency to effect focusing servo and tracking servo. As a result, resonance occurs in the actuator, and this has led to a case where servo is not applied.
For example, the gain frequency response characteristic in the focusing direction of the lens actuator shown in FIG. 1 is such as shown in FIG. 3 of the accompanying drawings. In this figure, f.sub.1 is the resonant frequency of a support system for the movable portion, and is usually about 100 Hz. Also f.sub.2, is the resonant frequency by the deformation of the movable portion, i.e., the lens holder. If a range partitioned by line a is prescribed as the necessary condition of the servo system, the peak f.sub.2, is over the line a, and if servo is applied in this state, the actuator will oscillate and become unusable. This line a is determined by the amount of surface warp, the amount of eccentricity and the number of revolutions of the disk.
FIG. 4 of the accompanying drawings is a view in which the deformation of a bobbin at f.sub.2, has been simulated. As is apparent from this, the surrounding annular portion is deformed in such a manner as to open outwardly, and correspondingly thereto, the portion holding the objective lens is also deformed. This can be structurally inferred as a result of the fact that the opening 102a in which the internal yoke is inserted is large and therefore the rigidity of the lens holder 102 is reduced.